The discovery of metallocene catalysts activated with alumoxanes has enabled the synthesis of new polyolefins with improved properties. However, metallocene produced polymers are known to have targeted properties, such as narrow molecular weight distributions, which can lead to uneven product properties, e.g., good strength but poor processability.
Thus, there is a need for catalyst systems, particularly metallocene catalyst systems, that are capable of producing polymers having multimodal properties, such as molecular weight distribution, in a variety of polymerization processes.
There is also a need for catalyst systems comprising a single catalyst precursor that are capable of producing olefin polymers having bimodal molecular weight distribution via one-reactor polymerization.
Traditional methods to obtain multimodal molecular weight distributions include, using a single-catalyst system to perform multiple polymerization in a sequential polymerization processes, as well as a reactor system using differing hydrogen concentrations in different reactors. Likewise, using a multi-catalyst system to perform polymerization in a single reactor has also been used. However, continuous single reactor operation is difficult and often requires trim catalyst, which is usually one of the catalysts in the multi-catalyst system added to the polymerization reactor during the polymerization process to compensate for differing, activities, lifetime, etc., of the different active catalyst species which is neither cost effective nor operation friendly, for example, see U.S. Pat. No. 8,318,872.
WO 03/051934 discloses an alternative form of catalyst which is provided in solid form but does not require a conventional external carrier material such as silica. The alternative form is based on the finding that a homogeneous catalyst system containing an organometallic compound of a transition metal can be converted, in a controlled way, to solid, uniform catalyst particles by first forming a liquid/liquid emulsion system, which comprises a homogeneous solution of catalyst components as the dispersed phase, and as the continuous phase solvent where the catalyst solution is as dispersed droplets therewith, and then solidifying said dispersed droplets to form solid particles comprising the said catalyst.
Additional references of interest include: Pullukat, T. J., et al., “Microspherical Silica Supports with High Pore Volume for Metallocene Catalysts,” presented at Metallocenes Europe '97, Dusseldorf, Germany, Apr. 8-9, 1997, pp. 1-11; U.S. Pat. No. 6,001,764; Nello Pasquini (Ed.), Polypropylene Handbook, 2nd Edition, Hanser Publisher, Munich, 2005, pp 361-380; U.S. Pat. Nos. 8,729,206; and 6,001,764.
Thus, there is a need for a single-catalyst system that can be run in a continuous single reactor system (such as a gas phase reactor system), to produce multimodal polymer products. It is particularly desirable that the system operate well under constant reaction conditions in a continuous process (such one having continuous monomer feeds and or continuous polymer withdrawal) to produce multimodal (such as multimodal molecular weight distribution) polymer products to reduce cost and simplify operation.